## create_cases_users.rb
class CreateCasesUsers < ActiveRecord::Migration
  def self.up
    create_table :cases_users do |t|
		t.column :case_id, :integer
		t.column :user_id, :integer

      t.timestamps
	end

	add_index :cases_users, [:case_id, :user_id]
	add_index :cases_users, [:user_id, :case_id]
  end

  def self.down
    drop_table :cases_users
  end
end

## user.rb
class User < ActiveRecord::Base
  include Authentication
  include Authentication::ByCookieToken

  has_many :cases_users
  has_many :cases, :through => :cases_users

  attr_accessor :answer, :password, :login, :old_password, :case_ids
  after_save :update_cases

  #after_save callback to handle case_ids
  def update_cases
		unless case_ids.nil?
			self.cases_users.each do |m|
				m.destroy unless case_ids.include?(m.case_id.to_s)
				case_ids.delete(m.case_id.to_s)
			end
			case_ids.each do |g|
				self.cases_users.create(:case_id => g) unless g.blank?
			end
			reload
			self.case_ids = nil
		end
  end

## case.rb
class Case < ActiveRecord::Base
	has_many :cases_users
	has_many :users, :through => :cases_users
	# end relevant code
end

## cases_users.rb
class CasesUsers < ActiveRecord::Base
	belongs_to :user
	belongs_to :case
end

## cases_controller.rb
  def add
	@case = Case.find(params[:id])
	@user = User.find(session[:user].id)
	@case << @user

	redirect_to :controller => :cases, :action => :my_cases
  end